1. Field of the Invention
The present invention generally relates to video cameras and, more particularly, is directed to an optical system including image-pickup lenses and so on.
2. Description of the Prior Art
A video camera is generally used to convert an optical image arriving at an image pick-up plane of an imager means such as a solid state imager element (hereinafter referred to as a CCD (charge-coupled device)) or the like through an imager lens to an electrical video signal. For limiting the amount of light arriving at the image pick-up plane of the imager means within an appropriate constant range, an iris composed of a plurality of diaphragms is provided in front of or behind the imager lens. The amount of light arriving at the image pick-up plane of the imager means such as a CCD or the like can be adjusted to be substantially constant by changing an aperture formed by the iris in accordance with the amount of incident light.
However, the imager lens inherently implies a phenomenon generally called vignetting by which a peripheral portion of a picked-up image becomes darker than a central portion thereof. FIGS. 1 and 2 are diagrams for explaining the vignetting. As shown in FIG. 1, lights incident with angles .theta..sub.1 and .theta..sub.2 deviated from the central axis OA of an optical image to be taken are partially intercepted by the edges of lenses L, whereby cross-sections a, b, c of effective luminous fluxes become smaller as the inclination angles thereof relative to the central axis OA increase and then arrive at an image pick-up plane F. Therefore, the cross-section c of the effective luminous flux arriving at a peripheral portion of the image pick-up plane F becomes extremely smaller than the cross-section a of the effective luminous flux arriving at a central portion of the same, as shown in FIG. 2, whereby the peripheral portion becomes darker than the central portion. The amount of the vignetting may vary depending on an opening formed by the iris so that the lenses must be designed so as to reduce the occurrence of the vignetting.
In recent years, the sensitivity of the imager means such as CCD or the like employed in video cameras tends to be enhanced so that, if the iris is not closed almost completly, then an appropriate exposure cannot be obtained when the cameraman takes a picture outdoors. However, if the iris is largely closed, the aperture of the iris becomes a pin-hole shape, thereby incurring the inconvenience such that the resolution of a picked-up image is degraded due to the diffraction of light.
For this reason, a neutral density (ND) filter for limiting the amount of a passing light is arranged on an optical path through which an optical image arrives at the image pick-up plane, such that the amount of a passing light is limited by this ND filter to prevent the iris from being excessively closed (see Japanese Patent Application No. 57-100294). The ND filter reduces the light amount without changing the hue or the like.
The ND filter is attached on any one of the diaphragms constituting the iris such that the ND filter is positioned on the optical path in association with opening and closing movements of the iris, so that the ND filter is moved onto the optical path only when the iris is closed is being closed
Nevertheless, the ND filter attached on a diaphragm constituting the iris is moved in association with opening and closing movements of the iris, which results in an inconvenience that inconsistent lighting due to the above-mentioned vignetting becomes extremely prominently depending on the position of the ND filter.
More specifically, as, for example, shown in FIG. 3, suppose that an iris is composed of two diaphragms 2, 3 for narrowing an optical path 1 which are respectively moved in the directions indicated by arrows m, n. The diaphragms 2, 3 are arranged, for example, between two of a plurality of lenses L, as shown in FIG. 1. An ND filter 4 is attached on the diaphragm 3 such that the ND filter 4 is positioned on the optical path 1 by moving the diaphragm 3.
FIG. 4 shows that the ND filter 4 is positioned on the optical path 1 by closing the iris. In the condition that the iris is closed substantially by half, as shown in FIG. 4, there remains a transparent portion 5 at a position deviated from the center of the optical path 1 which is not covered with the ND filter 4.
If the transparent portion 5 remains at a position deviated from the center of the optical path 1 as described above, then vignetting stands out prominently.
FIG. 5 shows the occurrence of vignetting in such a case, i.e., cross-sections a', b', c' of effective luminous flux in the present example (in the condition where the iris is positioned as shown in FIG. 4) corresponding to the cross-sections a, b, c of effective luminous flux arriving at the image pick-up plane F shown in FIG. 2. The cross-section a' of the effective luminous flux arriving at the center of the screen includes a light l passing through the transparent portion 5 on the left side thereof and a light passing through the ND filter on the remaining portion (the hatched area). As for each of the cross-sections b', c' deviated from the center, a light passing through the transparent portion 5 does not arrive due to vignetting, whereby the light passing through the ND filter only arrives. Therefore, a central portion of the imager screen becomes very bright because of the light arriving through the transparent portion 5, while a peripheral portion becomes dark due to the limited light amount by the ND filter, whereby an image whose central portion only is bright is taken. Such a condition occurs when the iris is half closed. However, such a half closed iris is frequently used in actual shooting with a video camera, so that images taken in the above-mentioned condition stand out. If the difference in light amount between the central portion and the peripheral portion of an imager screen is small, it is possible to make the difference inconspicuous to some extent by signal processing performed by a picked up signal processing circuit. However, the light amount on the peripheral portion of the imager plane may sometimes be, for example, approximately 1/4 that of the central portion. In such a case, it is not possible to completely correct such difference by means of a picked up signal processing circuit.